On the first 6 quizzes of his geometry class, Omar got an average score of 77. What does he need on the next quiz to have an overall average of 79?
Let his score on the next quiz be $x$ The sum of all of his scores is then $6 \cdot 77 + x$ The same sum must also be equal to $7 \cdot 79$ Solve: $x = 7 \cdot 79 - 6 \cdot 77 = 91$.